Treatment of lignocellulosic substrates with ozone

ABSTRACT

The invention relates to a process for the preparation of lignocellulosic substrates having a high digestibility. This process consists in bringing said lignocellulosic substrates, which have been ground beforehand and optionally moistened in the case of dry substrates, in a stirred reactor, into contact with ozone produced from a carrier gas, the ozone being present in the carrier gas in a concentration of between 80 and 200 g/m 3  NTP, with a residence time of between 8 and 40 min in said reactor.  
     The invention further relates to the lignocellulosic substrates treated in this way.

[0001] The present invention relates to a novel process for thepreparation of lignocellulosic substrates for animal nutrition whichhave a high digestibility.

[0002] It further relates to the lignocellulosic substrates obtainableby carrying out this process.

[0003] The invention applies especially to the field of the treatment ofproducts originating from agriculture and the agri-foodstuffs sector,and to the industrial manufacture of animal feedingstuffs.

[0004] Lignocellulosic substrates as such are rather unsuitable foranimal nutrition because of their low digestibility due to the presenceof lignin.

[0005] Various processes have been proposed for increasing theirdigestibility, two examples being hydrolysis under the action ofpressurized steam (STAKE process) and attack with hydrogen peroxide inthe presence of cyanamides (FR-A-2 560 898). However, these processeshave practical difficulties and are expensive, which explains why theyhave not yet been exploited industrially.

[0006] Patent application FR-A-2 603 775 proposes the treatment ofcereal straws and other ground, dry lignocellulosic materials with a gascontaining ozone in a sufficient amount for between 10 and 40 grams ofozone to be absorbed per kilogram of dry straw. The process described insaid document consists in passing said gas through a bed of straw orother lignocellulosic substrate, thereby making it possible to increasethe digestibility with an upper limit of about 10%. By way of comparisonand according to patent application FR-A-2 603 775, the process appliedto an aqueous suspension of straw (moisture content of 500%) affords avery small increase in digestibility, being of the order of a fewpercent.

[0007] However, the process described in patent application FR-A-2 603775 has the following disadvantages:

[0008] the digestibility of the dry straw does not exceed a certainupper limit and decreases even when the degree of ozonization is greaterthan 25 milligrams of ozone per gram of dry straw;

[0009] the relatively small increase in digestibility has to becompensated by the addition of a nitrogen compound;

[0010] the process seems to be suitable only for the treatment of asmall amount of lignocellulosic substrate;

[0011] the use of a fixed bed reactor does not allow the substrate,finely divided though it is, to present a maximum surface area to thechemical reaction;

[0012] the use of an amount of ozone determined solely by the amountwhich reacts with the lignocellulosic substrates does not afford goodcontrol over the operating conditions; and

[0013] the poor yield of the system gives rise to an overconsumption ofozone for a limited result.

[0014] Canadian patent no. 1 169 794 proposes a process for thetreatment of lignocellulosic substrates with ozone which comprisesgrinding said substrate to a size of at most 4 mm, mixing said groundsubstrate with water in a ratio of 1:10 to 10:1 and then exposing theresulting mixture to ozone, the concentration of ozone in the carriergas being between 0.75 and 6.8%.

[0015] When used on small amounts of substrate in the order of 20 g,this process affords an acceptable increase in digestibility. However,it has the following two disadvantages:

[0016] on the one hand the residence time in the ozone treatment reactorhas to be at least one hour and preferably 16 hours, which amounts to arelatively long time for an industrial process; and

[0017] on the other hand the amount of water used is such as to producea suspension of material, so the finished product has to be subjected toa subsequent drying treatment in order to be in an acceptable form forthe animals for which these finished products are intended.

[0018] It is interesting to note the divergence of results between thesetwo patents, the one advocating the treatment of dry substrates withozone and the other advocating the treatment of aqueous suspensions ofsubstrates with ozone.

[0019] The Applicant has now found, surprisingly, that the abovedisadvantages can be overcome by a process that consists in treatinglignocellulosic substrates, which have been ground beforehand andoptionally moistened (without creating an aqueous suspension) when thesubstrate is dry, e.g. straw, in a stirred reactor, with ozone producedby a carrier gas, the ozone being present in the carrier gas in a givenconcentration, with a greatly reduced residence time in said reactor.

[0020] Thus, according to a first feature, the invention relates to aprocess for the preparation of lignocellulosic substrates having a highdigestibility, which consists in bringing said substrates, which havebeen ground beforehand and optionally moistened when the substrate isdry, in a stirred reactor, into contact with ozone produced from acarrier gas, the ozone being present in the carrier gas in aconcentration of between 80 and 200 g/m³ NTP and preferably of between140 and 160 g/m³ NTP, for a residence time of between 8 and 40 min andpreferably of between 15 and 30 min in said reactor.

[0021] This process makes it possible for the first time, and in anentirely satisfactory manner, to solve the problem of insufficient andlimited digestibility of relatively dry lignocellulosic substrates in agreatly shortened reaction time, and to obtain hitherto unavailablelignocellulosic substrates having an acceptable moisture content forstorage and animal nutrition and a high digestibility.

[0022] According to a second feature, the invention relates to thelignocellulosic substrates obtainable by the above process.

[0023] By way of example, with the process of the invention applied towheat straw (moisture content of 4-5%), which normally has an initialdigestibility of about 40%, it is possible to increase thisdigestibility by 20% or more by means of a treatment with ozone in anamount of 3 kg of ozone per tonne of straw at an ozone concentration of115 g/m³ NTP, for a contact time of 20 to 30 min. Likewise, for sugarcane bagasse, which has a mean digestibility of 20% in the naturalstate, it is possible to raise this digestibility to 40% or more bymeans of a treatment with ozone in an amount of 3 kg of ozone per tonneof bagasse, and to 60% for a treatment rate of 9 kg of ozone per tonneof bagasse, at an ozone concentration of 122 g/m³ NTP, for a contacttime of 20 to 30 min.

[0024] In the process of the invention, the concentration of ozone inthe carrier gas is between 80 and 200 g/m³ NTP (Normal Temperature andPressure) and preferably between 140 and 160 g/m³ NTP. If thisconcentration is below 80 g/m³ NTP, the digestibility of the ozonizedsubstrate is not sufficiently high to be of value for direct animalnutrition. If the ozone concentration is above 200 g/m³ NTP, it ispossible to observe a degradation of the lignocellulosic substrate suchthat it becomes difficult to transport and use. The carrier gasadvantageously consists of oxygen. The carrier gas can also be producedfrom ambient air which has been filtered, compressed and dried at thedew point of between −50 and −70° C. In another alternative, the carriergas can consist of a mixture in any proportions of pure oxygen andfiltered, compressed and dried air.

[0025] According to one particular characteristic, the relative pressureof the ozonized carrier gas when it is brought into contact with saidsubstrates is between 0.6 and 1.5 bar and preferably between 0.6 and 1.2bar. If the pressure is below 0.6 bar, the problem of poor penetrationof the ozone into the lignocellulosic substrate may arise, resulting inan imperfect treatment and a small increase in digestibility. On theother hand, if the pressure is above 1.5 bar, the following problems mayarise:

[0026] as regards the generation of the ozone: industrial ozonizers donot operate at pressures above 1.5 bar;

[0027] as regards the treated substrate: above this pressure, the ozonepenetrates the lignocellulosic material very deeply and, because of itsreactivity, almost completely destroys the structure of the material tobe treated.

[0028] The lignocellulosic substrates used in the present inventioninclude any substrate originating from the agri-foodstuffs sector. Thesubstrates can be classed in two categories, namely dry substrates andmoist substrates. Examples of dry substrates include rye, wheat andalfalfa straws, sugar beet cossettes and cereal grains, and examples ofmoist substrates include sugar cane bagasse, sorghums, ray grasses andhays.

[0029] The amount of lignocellulosic substrate treated can be as much asseveral tens of tonnes per hour because of the high concentration ofozone used and the specificity of the process developed, so the processof the invention is suitable for use on an industrial scale.

[0030] If the substrate is dry, it has to be moistened to optimize theresult. This moistening consists in spraying with water or ozonizedwater in an amount of between 1 and 6% and preferably of between 1 and5%, based on the dry matter. If the moisture content is greater than 6%,the digestibility will be reduced.

[0031] The substrate also has to be ground before being treated withozone. This operation makes it possible to improve the packing of thereactor, avoiding preferential passages, to increase the reactivesurface area of the substrate and to favor the homogeneity of mixing.The grinding is carried out by processes well known to those skilled inthe art, before moistening and before the substrate enters the stirredreactor.

[0032] According to one particular characteristic, the ground substrateshave a size of between 5 mm and 20 mm, the range from 10 to 20 mm beingparticularly preferred. If the size is less than 5 mm, the substrate canbe in the form of powder in the case of dry materials and in the form ofslurry in the case of moist substrates, these forms being unacceptable.If the size is greater than 20 mm, treatment is difficult and it ispossible to observe preferential passages and voids in the packing.

[0033] The treatment with ozone is carried out in a stirred reactor,which will hereafter be referred to as an ozonization reactor. Thisreactor can be of vertical or horizontal type.

[0034] In general, the ozonization reactor used can consist of acylindrical body comprising a device for introducing the groundlignocellulosic substrate, such as a hopper, and an internal device formixing the lignocellulosic substrate and assuring its residence time inthe reactor so as to optimize the treatment with ozone. The groundlignocellulosic substrate can be brought into contact with the ozonizedcarrier gas continuously or batchwise in said ozonization reactor.

[0035] The residence time in the ozonization reactor is between 8 and 40min and preferably between 15 and 30 min. If this residence time is lessthan 8 min, the treatment with ozone is insufficient. If it is greaterthan 40 min, degradation of the lignocellulosic structure is observed.

[0036] Thus, in contrast to the known processes of the prior art, theprocess of the invention makes it possible to treat an amount oflignocellulosic substrate that is suitably large for use on theindustrial scale, in a very rapid reaction time.

[0037] The constituent materials of the body of the ozonization reactorwill be chosen so as to assure abrasion resistance and resistance to theoxidation generated by the presence of ozone in high concentration. Sucha material can be e.g. a stainless steel known to those skilled in theart.

[0038] The mixing device with which the ozonization reactor is equippedcan be any device known to those skilled in the art. This device mustallow homogeneous mixing of the ground substrate while at the same timeallowing its transport towards the outlet of the ozonization reactor orits internal recycling, depending on whether the reaction is beingcarried out continuously or batchwise.

[0039] The speed of rotation of said mixing device and the dimensions ofthe reactor used will be calculated by those skilled in the art as afunction of the amount of material to be treated, the ozoneconcentration used and the intended residence time. In one preferredembodiment, to treat 3 tonnes of ground lignocellulosic substrates withan ozone concentration of 150 g/m³ NTP in a reactor having a diameter of1.20 m and a total height of 4.60 m, the speed of rotation is in theorder of 100 to 120 rpm.

[0040] Examples of mixing devices include an Archimedean screw, aconical screw or a drive shaft on which blades are mounted.

[0041] If this last type of device is used, namely a drive shaft andblades, the distance between the last blades and the bottom of theozonization reactor must be sufficient to prevent the mass to be treatedfrom building up at the bottom of said reactor. Those skilled in the artwill be capable of determining such a distance as a function of thedensity of the reacting mass. In the case of vertical ozonizationreactors, a scraping and mixing device can be added at the conical end.

[0042] In the case of horizontal ozonization reactors, the mixing deviceis centered in said reactor. It is advantageous to use a drive shaftcarrying alternate sets of blades of two different dimensions, locked at120° on the horizontal shaft. Each set of blades consists of 3 blades ofthe same length and of large diameter, the following set consisting of 3blades of the other length (smaller diameter), offset by 60° relative tothe blades of the first set. These small blades can carry a mixingdevice, inclined by a few degrees to the vertical axis, so as to ensurethat the substrate undergoes a helical rotational movement duringtreatment. This movement further ensures that the material to be treatedundergoes a horizontal translational movement with permanent renewal ofthe interface.

[0043] In the case of vertical ozonization reactors, the mixing deviceis either centered on the vertical axis of said reactor, or offsetrelative to this axis and driven with a gyratory movement around thisvertical axis. In the latter case, it is preferable to use a conicalscrew. The axis of rotation of the screw is at an angle relative to thevertical axis of the ozonization reactor. The upper part of the screw isconnected to a drive device via gearboxes and the lower part is centeredon the vertical axis at the end of said reactor. The screw rotates aboutitself to assure mixing and rotates about the vertical axis to cover thetotal volume of substrate to be treated. Depending on the speed ofrotation of the screw about the vertical axis of the ozonizationreactor, the movement described can be likened to a simple cycloid or toa hypocycloid of shortened pitch.

[0044] The mixing device is driven by an electromechanical device,making it possible to adjust the speed of the mixing device so as toassure the intended residence time in the ozonization reactor withprecision. Preferably, the mixing device is driven by a variable-speedmotorized step-down gear unit.

[0045] The electromechanical device itself possesses one or twocentering and sealing devices well known to those skilled in the art.

[0046] In the case of dry lignocellulosic substrates requiringmoistening, the ozonization reactor is also equipped with a device formoistening the reacting mass. An example of such a device comprisesspray nozzles. The moistening rate will be determined as a function ofthe amount of water necessary to achieve the chosen moisture content.Those skilled in the art will easily be capable of determining such arate.

[0047] The ozonization reactor is also equipped with one or more devicesfor injecting the ozonized carrier gas, said gas itself being producedfrom an ozonizer well known to those skilled in the art. The device(s)for injecting the ozonized carrier gas are designed so as to ensure thedistribution of the gas in said ground substrate with a sufficientinjection rate to assure a good penetration of said gas into saidsubstrate. Injection nozzles are examples of such devices. In general,the injection rate of the ozonized carrier gas is between 30 and 60m.s⁻¹.

[0048] In the case of horizontal ozonization reactors possessing asingle device for injecting the ozonized carrier gas, said device isplaced in the upstream part of said reactor, just behind the device forintroducing the ground substrate and the moistening device. If this typeof reactor has several devices for injecting the ozonized carrier gas,said devices can be placed over the whole length of the mixing device,for example between the blades. In general, these injection devices willbe arranged at a distance of between 0.08 d and 0.15 d, where d is thediameter of the reactor. The number of injection devices will be chosenso that the injection rate falls within the range indicated above. Suchan arrangement affords better control over the ozone concentration used.

[0049] In the case of vertical ozonization reactors, the devices forinjecting the ozonized carrier gas are placed in the lower part of saidreactor so that their action is of the countercurrent type.

[0050] The ozonization reactor is also generally equipped with a devicefor discharging the ozonized carrier gas after the reaction to the upperpart of the ozonization reactor (residual ozone present in the carriergas after the reaction) and with a pressure measuring device, both saiddevices being well known to those skilled in the art.

[0051] Finally, the ozonization reactor is equipped with a device fordischarging the treated product, located opposite the device forintroducing the ground ligno-cellulosic substrate. This dischargingdevice can consist e.g. of apertures in the flat end opposite the feed.In the case of a continuous reaction, the device for discharging theproduct and the device for introducing the ground lignocellulosicsubstrate must be leaktight. An alveolar slide valve is an example ofsuch a device.

[0052] Furthermore, as the ozonization reaction is of exothermic type,the body of the ozonization reactor is usually equipped with atemperature control device and a cooling device for maintaining aconstant temperature inside said reactor and in the reaction medium,without a vertical or radial temperature gradient, for the requisitereaction time.

[0053] This efficient cooling of the ozonization reactor favors the safeoperation of said reactor and affords precise control over theozonization reaction.

[0054] The cooling device can be located on the outside or inside of theozonization reactor. It can be fed e.g. with cold water under pressureor via a circuit of iced water produced by a refrigerating set. The bodyof the ozonization reactor is advantageously cooled by means of anexternal cooling device, which can consist e.g. of a conventional deviceof the water jacket type or a circuit made up of half-shells thatcaptures the heat flux originating from the reaction mass and dissipatesit to the outside.

[0055] The lignocellulosic substrate treated with ozone in theozonization reactor will be collected e.g. in a hopper and then passedon for bagging, weighing or any other packaging operation prior todistribution. This gives a ready-to-use finished product with a highdigestibility, so there is no need for complements, said product beingintended for animal nutrition and especially for feeding cattle, sheepand poultry.

[0056] The invention will be understood more clearly from the Examplesand attached diagrams, which are given solely by way of illustration andwithout implying a limitation. In said diagrams:

[0057]FIG. 1 is a curve showing the variability of the digestibility invitro as a function of the treatment rate of the moistened, drylignocellulosic substance (straw) with ozone in a dynamic (stirred)reactor and in a static reactor; and

[0058]FIG. 2 is a curve showing the variability of the digestibility invitro as a function of the treatment rate of the moist lignocellulosicsubstrate (bagasse) with ozone in a dynamic (stirred) reactor and in astatic reactor.

EXAMPLE 1 Comparison of the Modification of the Digestibility of StrawTreated in a Dynamic Reactor and in a Static Reactor

[0059] Wheat straw was ground to a size of the order of 13 mm. Saidground straw, having various moisture contents as a result of priormoistening, was then treated with an ozonized carrier gas, either in astatic reactor (R. St), i.e. a reactor containing a fixed bed of strawthrough which a stream of ozone borne by the oxygen carrier gas waspassed from bottom to top, or in a dynamic (or stirred) reactor (R.Dym), i.e. a vertical or horizontal reactor in which the mixing devicewas a shaft and blades (horizontal reactor) or a central screw withrecirculation (vertical reactor), the residence time in the reactorsbeing 20 to 30 min.

[0060] 5 experiments with the following characteristics were performed:

[0061] x: ozone concentration (CO₃) of 115 g/m³ NTP, straw dryness of89.6%, dynamic reactor,

[0062] : ozone concentration of 111.5 g/m³ NTP, straw dryness of 85%,static reactor,

[0063] ∘: ozone concentration of 47 g/m³ NTP, straw dryness of 85%,static reactor,

[0064] ∇: ozone concentration of 47 g/m³ NTP, straw dryness of 55%,static reactor, and

[0065] ?: ozone concentration of 115 g/m³ NTP, straw dryness of 55%,static reactor.

[0066] After treatment, the digestibility in vitro (DIV) was determinedby the method of Tillery and Terry in percent as a function of thetreatment rate with ozone, itself expressed in percent of ozone relativeto dry matter (3% means that 30 g of ozone were used to treat 1000 g ofsubstrate).

[0067] The results of these tests are given in the form of curves inFIG. 1.

[0068] The results obtained lead to the following two conclusions:

[0069] Firstly, in the case of static reactors, increasing the ozoneconcentration, in the present case from 47 g/m³ to 115 g/m³, with adryness of 55%, brings about only a very small variation in thedigestibility of the straw, which is in agreement with the resultsobtained according to patent application FR-A-2 603 775.

[0070] Secondly, when using a dynamic reactor and a high ozoneconcentration (115 g/m³), the digestibility results improve as afunction of the treatment rate applied.

[0071] These results eloquently prove the increase in digestibilityprovided that the operating parameters are perfectly controlled.

EXAMPLE 2 Comparison of the Modification of the Digestibility of BagasseTreated in a Dynamic Reactor and in a Static Reactor

[0072] The procedure of Example 1 was repeated except that thelignocellulosic substrate used was bagasse and the following 3experiments were performed:

[0073] x: ozone concentration of 112 g/m³ NTP, dynamic reactor,

[0074] : ozone concentration of 111.8 g/m³ NTP, static reactor,

[0075] ∘: ozone concentration of 46 g/m³ NTP, static reactor.

[0076] The results of these tests are given in the form of curves inFIG. 2.

[0077] The results obtained lead to the same conclusions as thoseindicated in Example 1.

EXAMPLE 3 Improvement in the Digestibility of Wheat Grains Treated withOzone

[0078] The digestibility of wheat grains treated with ozone was analyzedon cockerels by measuring the metabolizable energy (ME) in accordancewith farming procedures SVDC 03 and SVDM 01. The method applied is therapid method of measuring metabolizable energy developed by Mr Lessireof INRA and the method of determining the viscosity of aqueous extracts(Viscometer method, standard NFV 03749).

[0079] Groups of 9 cockerels each were fed for 8 days with 4 differentbatches of untreated wheat grains or wheat grains treated with ozone, asfollows:

[0080] batch 1: untreated grains

[0081] batch 2: grains treated with 3 g of ozone per kilogram of grains(85 g/m³ NTP) with a moisture content of 4%

[0082] batch 3: grains treated with 4 g of ozone per kilogram of grains(85 g/m³ NTP) with a moisture content of 4%

[0083] batch 4: grains treated with 5 g of ozone per kilogram of grains(85 g/m³ NTP) with a moisture content of 4%

[0084] The wheat grains corresponded to 97% of the food ration.

[0085] The raw results are given in the Table below, in which:

[0086] UV denotes useful viscosity,

[0087] RE denotes raw energy,

[0088] DM denotes dry matter. TABLE Sample 1 Sample 2 Sample 3 Sample 4Moisture content 12.6 15.5 15.5 15.4 (%) Raw protein (%) 10.32 9.9210.04 10.09 Fats (%) 1.38 1.36 1.33 1.42 Raw cellulose (%) 2.06 1.961.76 1.75 Minerals (%) 1.49 1.43 1.38 1.38 Starch (%) 60.39 58.81 58.1458.01 Sugars (%) 2.19 2.75 2.75 2.88 Wall (%) 10.54 9.54 9.59 8.91 Spec.visc. 0.30 0.40 0.50 0.40 (cPs/g sec) UV pentosan 1.80 2.40 2.40 2.20(Log/sec) RE (kCal/kg DM) 4369 4354 4369 4347 ME (kCal/kg DM) 3604 ± 263597 ± 26 3578 ± 12 3618 ± 14 ME/RE (%) 82.5 82.6 81.9 83.2

[0089] Analysis of the Results

[0090] In the context of poultry nutrition, two parameters are to beconsidered: viscosity and metabolizable energy (ME). Viscosity isconsidered to be a disadvantage and represents a good factor foranalyzing the digestibility.

[0091] The effect of treating the grains with ozone was to cause a fewsmall but significant variations in composition: a slight degradation ofthe raw cellulose and of the starch composition and the walls, togetherwith an increase in the free sugars.

[0092] This degradation is verified by both the specific viscosity andthe pentosan viscosity. In fact, a slight degradation of thepolysaccharides, specifically the pentosans, initially causes anincrease in the viscosity due to solubilization of the pentosans and theshort cellulose fragments.

[0093] If the oxidation is increased, a drop in viscosity can beanticipated. This drop appears as from 5 g of ozone per kg of grains.

[0094] This analysis is confirmed by the fact that the ME increases inreal terms as from 5 g of ozone per kg of grains, i.e. as from sample 4.

1. A method for preparing lignocellulosic substrates having highdigestibility, comprising the steps of: grinding said lignocellulosicsubstrates, moistening the lignocellulosic substrates in the case of drysubstrates, and bringing the lignocellulosic substrates in a stirredreactor, into contact with ozone produced from a carrier gas, the ozonebeing present in the carrier gas in a concentration of between 80 and200 g/m³ NTP, wherein the residence time in said reactor is between 8and 40 min, and wherein the moisture content of said substrates isbetween 1 and 6% based on the dry matter.
 2. A method according to claim1, wherein the residence time is between 15 and 30 min.
 3. A methodaccording to claim 1, wherein the concentration of ozone in the carriergas is between 140 and 160 g/m³ NTP.
 4. A method according to claim 1,wherein the moisture content of the lignocellulosic substrates isbetween 1 and 5%, based on the dry matter.
 5. A method according toclaim 1, wherein the size of the ground lignocellulosic substrates isbetween 5 mm to 20 mm.
 6. A method according to claim 1, wherein therelative pressure of the ozonized carrier gas when it is brought intocontact with said substrates is between 0.6 and 1.5 bar.
 7. A methodaccording to claim 1, wherein the lignocellulosic substrates areselected from the group consisting of dry substrates and moistsubstrates.
 8. A lignocellulosic substrate obtainable by the methodaccording to claim
 1. 9. A method according to claim 1, wherein the sizeof the ground lignocellulosic substrates is between 10 mm to 20 mm. 10.A method according to claim 1, wherein the relative pressure of theozonized carrier, gas when it is brought into contact with saidsubstrates is between 0.6 and 1.2 bar.
 11. A method according to claim7, wherein the dry substrates are selected from the group consisting ofrye, wheat and alfalfa straws, sugar beet cossettes and cereal grains.12. A method according to claim 7, wherein the moist substrates areselected from the group consisting of sugar cane bagasse, sorghums, raygrasses and hays.